Active material for rechargeable lithium battery and rechargeable lithium battery comprising same

ABSTRACT

The present invention relates to an active material for a rechargeable lithium battery and a rechargeable lithium battery including the same. The active material includes an active material and a fiber-shaped or tube-shaped carbon conductive material attached to the surface of the active material. The active material includes a conductive shell including a fiber-shaped or tube-shaped carbon conductive material and increases discharge capacity due to improved conductivity and improves cycle-life efficiency by maintaining paths between active material particles during charge and discharge cycles.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.11/607,817, filed Nov. 30, 2006, which claims priority to and thebenefit of Korean Patent Application No. 10-2005-0115824, filed in theKorean Intellectual Property Office on Nov. 30, 2005, the entirecontents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to an active material for a rechargeablelithium battery, and a rechargeable lithium battery including the same,and more particularly, the present invention relates to an activematerial for a rechargeable lithium battery having excellent high ratecharacteristics and/or cycle-life characteristics, and including thesame.

(b) Description of the Related Art

In recent times, due to due to reductions in size and weight of portableelectronic equipment in accordance with developments in the electronicsindustries, such portable electronic equipment has increasingly beenused. A battery having a high energy density for a power source of sucha portable electronic equipment is needed and thus research intorechargeable lithium batteries has been actively made.

For a positive active material of a rechargeable lithium battery,lithium-transition element oxides have been used, and for a negativeactive material, carbon-based active materials, silicon, tin, or alloysalloyed with other metals have been used.

The above non-carbon-based active materials such as lithium-transitionelements oxide, or silicon, tin or alloys require a carbon-basedconductive material to provide a high-capacity battery due to their lowelectronic conductivity.

For the conductive material, carbon-based conductive materials such asconductive carbon black have generally been used.

Particularly, carbon black in the form of very small nano-beads isagglomerated and has a large specific area and has generally been usedfor the conductive materials. However, carbon black is an electrostaticagglomerate of nano beads and therefore when interfaces between activematerial particles are large in accordance with electrode expansionduring long cycles of charge and discharge, the agglomerate of carbonblack particles may easily be separated and the conductive pathsdecrease, causing resistance in a battery and resulting in reduction ofcycle-life. The conductive material has a sufficient contact with activematerial particles, and therefore, an increased amount of the conductivematerial is required.

SUMMARY OF THE INVENTION

One embodiment of the present invention provides an active material thatmay provide a rechargeable lithium battery having good cycle-lifecharacteristics and/or high conductivity even when a small amount ofconductive material is used.

Another embodiment of the present invention provides a rechargeablelithium battery having excellent rate and cycle-life characteristics.

According to an embodiment of the present invention, an active materialfor a rechargeable lithium battery is provided that includes an activematerial and a fiber-shaped or tube-shaped carbon conductive materialattached to the surface of the active material.

According to another embodiment of the present invention, a rechargeablelithium battery is provided that includes a positive electrode, anegative electrode, and an electrolyte. At least one of the positive andnegative electrodes includes the active material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a rechargeable lithium battery accordingto one embodiment of the present invention.

FIG. 2 is a SEM photograph of an electrode surface of the coin cellaccording to Example 1 of the present invention after 40 charge anddischarge cycles.

FIG. 3 is a SEM photograph of an electrode surface of the coin cellaccording to Example 2 of the present invention after 40 charge anddischarge cycles.

FIG. 4 is a SEM photograph of an electrode surface of the coin cellaccording to Comparative Example 1 after 40 charge and discharge cycles.

DETAILED DESCRIPTION

The present invention relates to an active material having highconductivity for a rechargeable lithium battery that includes an activematerial surface-treated by a conductive material.

An electrode of a rechargeable lithium battery is generally fabricatedas follows: an active material slurry including an active material, aconductive material, and a binder are mixed in an organic solvent, andthen the slurry is coated on a current collector. The electrodefabricated as above includes a current collector, and an electrodeactive mass layer including the above positive active material, aconductive material, and a binder disposed on a current collector.

For the conductive material, carbon black in the form of very small nanobeads is agglomerated and has a large specific surface area. However,the carbon black is an electrostatic agglomerate and thus the conductivenetwork of the carbon black particles may be easily broken by electrodeexpansion resulting in an increase of cell resistance. In addition, asufficient electronic conductive path is not realized at a particlesurface and therefore a large amount of conductive material is requiredto obtain intended charge and discharge capacities.

On the contrary, according to one embodiment of the present invention, aconductive material is directly coated on a surface of an activematerial and thereby, the surface conductivity is improved. Theconductive material includes a fiber-shaped or a tube-shaped carbonconductive material.

For a conductive material, a conventional carbon nanotube has beensuggested in Implementation of a Thick-Film Composite Li-IonMicrocathode Using Carbon Nanotubes as the Conductive Filler (Qian Linand John N. Harb, Journal of the Electrochemical Society, 151 8A1115-A1119 2004). The disclosure relates to a microbattery, which isdifferent from a rechargeable lithium battery of the present inventionin terms of cell structure and operating mechanism.

In the case of a microbattery, compression is performed to ensureconductivity through a spot contact of conductive material particlessuch as graphite or carbon black. However, when a fiber-shapedconductive material such as carbon nanotube is used, compression neednot be performed since the fiber-shaped conductive material contactswell even without compression. In a rechargeable lithium battery,compression is needed in order to obtain a high density battery and thusprovide a high-capacity battery. In addition, the compression may beneeded to obtain an electrode with an appropriate thickness. However,for the microbattery as disclosed in the above article, compression isnot necessarily required if conductivity is ensured.

Furthermore, in the microbattery, a positive active material should bepulverized to a fine powder. Therefore, the specific surface area (BET)of a positive electrode is larger, and an excess amount of carbonnanotube is required. However, the excess amount of carbon nanotubecannot provide an electrode having a thickness of more than 100 μm. Whenthe electrode has a thickness of 100 μm or more, sufficient adherence isnot ensured even though a binder is used in an amount of 15 wt % ormore, and cracks can form on the surface of the electrode.

As described above, the microbattery is thoroughly different from arechargeable lithium battery in terms of a structure and operation.Furthermore, the purpose of using carbon nanotube and the effect thereoffor a microbattery are different from those of the present invention.Therefore, the disclosed article fails to suggest the present inventionto a person of ordinary skill in this art.

In Journal of Power Sources 119121 2003 770773 Roberto Dominko, apositive active material is disclosed including lithium cobalt oxide onthe particle surface to which carbon black is attached to improveconductivity. The surface conductivity of positive active material mayalso be improved. However, it is expected that spot contact may beseparated by electrode expansion during long charge and discharge cyclesand thereby conductivity decrease cannot be suppressed.

The active material for a rechargeable lithium battery according to thepresent invention includes an active material and a fiber-shaped ortube-shaped carbon conductive material attached to the surface of theactive material. The fiber-shaped or tube-shaped carbon conductivematerial may be attached to a portion of the active material surface orto the entire surface of the active material.

Such fiber-shaped or tube-shaped carbon conductive material is mixedwith the active material and attached to the surface of the activematerial by a process that is simpler than a method of growing afiber-shaped or tube-shaped carbon conductive material from the surfaceof the active material. In order to grow a fiber-shaped or tube-shapedcarbon conductive material, heat-treatment should be performed under anatmosphere without oxygen. However, a non-carbon-based active materialsuch as a metal or metal oxide undergoes deterioration due to acomposition change thereof when it is heat-treated under an atmospherewithout oxygen. According to the present invention, a heat treatmentprocess need not be performed and therefore the above problem does notoccur.

The fiber-shaped or tube-shaped carbon conductive material preferablyincludes carbon nano fibers or carbon nanotubes. When the carbonconductive material is not a fiber-shaped or tube-shaped material but apowder-type conductive material such as carbon black, surface cracks mayoccur and thus contact defects between particles may occur.

The fiber-shaped or tube-shaped carbon conductive material is oxidizedby acid-treatment during the manufacturing process. Such oxidizedfiber-shaped or tube-shaped carbon conductive material may suppressagglomeration during the coating of the active material and therebyimprove surface conductivity of particles compared to a non-oxidizedfiber-shaped or tube-shaped carbon conductive material.

The oxidized fiber-shaped or tube-shaped carbon conductive materialincludes functional groups selected from the group consisting ofcarboxyl groups (—COOH), hydroxyl groups (—OH), carbonyl groups (—COH),and combinations thereof that are attached to the surface thereof.

The fiber-shaped or tube-shaped carbon conductive material is present inan amount from 0.05 to 20 wt %, and preferably from 0.3 to 10 wt % basedon the total weight of the active material.

When the amount of the conductive material is less than 0.05 wt %,sufficient conductivity cannot provided, whereas when it is more than 20wt % active material adherence in the electrodes decreases.

The active material may include any positive or negative active materialthat is generally used for a rechargeable lithium battery. Theconductive shell improves conductivity of a non-conductivenon-carbon-based active material, and further improves conductivity of acarbon-based active material even more. Therefore, the conductive shelleffect of the present invention may be maximized in the case of anon-conductive non-carbon-based active material.

Such an active material may be selected from the group consisting ofcarbonaceous materials, materials capable of reversibly forminglithium-containing compounds by reaction with lithium ions, lithiumalloys, lithium-containing chalcogenide compounds, and combinationsthereof.

The carbonaceous material may include crystalline carbon, amorphouscarbon, or combinations thereof.

Materials capable of reversibly forming lithium-containing compounds byreaction with lithium ions include Si, Si oxide, Sn, Sn oxide, tin alloycomposites, transition element oxides, lithium metal nitrides, andlithium metal oxides represented by the following Formula 1:

Li_(x)M_(y)V_(z)O_(2+d)  Formula 1

where 0.1≦x≦2.5, 0<y≦0.5, 0.5≦z≦1.5, 0≦d≦0.5, and M is at least oneelement selected from the group consisting of Al, Cr, Mo, Ti, W, Zr, andcombinations thereof.

The lithium alloys may include an alloy of lithium and a metal selectedfrom the group consisting of Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra,Al, Fe, and Sn.

The lithium-containing chalcogenide compounds may be selected from thegroup consisting of compounds represented by the following Formulas 2 to16:

LiAO₂  Formula 2

LiMn₂O₄  Formula 3

Li_(a)Ni_(b)B_(c)M_(d)O₂ (0.95≦a≦1.1, 0≦b≦0.9, 0≦c≦0.5,0.001≦d≦0.1)  Formula 4

Li_(a)Ni_(b)Co_(c)Mn_(d)M_(e)O₂ (0.95≦a≦1.1, 0≦b≦0.9, 0≦c≦0.5, 0≦d≦0.5,0.001≦e≦0.1)  Formula 5

Li_(a)AM_(b)O₂(0.95≦a≦1.1, 0.001≦b≦0.1)  Formula 6

Li_(a)Mn₂M_(b)O₄ (0.95≦a≦1.1, 0.001≦b≦0.1)  Formula 7

DX₂  Formula 8

LiDS₂  Formula 9

V₂O₅  Formula 10

LiV₂O₅  Formula 11

LiEO₂  Formula 12

LiNiVO₄  Formula 13

Li_(3−x)F₂(PO₄)₃ (0≦x≦3)  Formula 14

Li_(3−x)Fe₂(PO₄)₃ (0.23 x≦2)  Formula 15

Li_(a)M′_(b)M″″_(c)(PO₄)_(d) (0<<3, 0<b+c≦2, 0<d<≦3)  Formula 16

where in Formulas 2 to 16:

A is selected from the group consisting of Co, Ni, Mn, and combinationsthereof,

B is Co or Mn,

D is Ti, Mo, or Mn,

E is selected from the group consisting of Cr, V, Fe, Sc, Y, andcombinations thereof,

F is selected from the group consisting of V, Cr, M, Co, Ni, Cu, andcombinations thereof,

M is at least one transition element or lanthanide metal selected fromthe group consisting of Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V, andcombinations thereof,

M′ and M″ are the same or different, and each is independently selectedfrom the group consisting of Fe, Co, Ni, Mn, Cu, V, Sn, Ti, Cr, andcombinations thereof, and

X is O or S.

Hereinafter, a method of preparing the active material of the presentinvention is described.

First, an active material to which a water-soluble polymer is attachedis added to a liquid of a fiber-shaped or tube-shaped carbon conductivematerial to obtain a mixture. The carbon conductive material is bound tothe water-soluble polymer attached to the active material so that thecarbon conductive material is bound to the surface of the activematerial.

The conductive material and active material are mixed in a weight ratioof 0.05 to 20:99.95 to 80. The concentration of the liquid of theconductive material may be controlled to be within an appropriate range.

The water-soluble polymer may be any water-soluble polymer havingviscosity such as gelatin, polyvinylalcohol, or cellulose-basedcompounds. The cellulose-based compounds include carboxyl methylcellulose, methyl cellulose, ethyl cellulose, hydroxy propyl methylcellulose, hydroxy propyl ethyl cellulose, or salts thereof. The salt ofthe cellulose-based compound may be salts of alkaline metals such as Na,K, or Li.

The active material to which a water-soluble polymer is attached may beprepared by dipping an active material in a liquid containing thewater-soluble polymer, and then drying the active material. An exampleof the liquid solvent for containing the water-soluble polymer is water,but it is not limited thereto. The concentration of the liquid of thewater-soluble polymer is not specifically limited, but the amount of thewater-soluble polymer is 5 wt % or less and preferably 2 wt % or lesswith respect to the weight of the active material. The amount of theactive material, and the drying process may appropriately be controlled.

The liquid of the fiber-shaped or tube-shaped carbon conductive materialis prepared by adding the fiber-shaped or tube-shaped carbon conductivematerial to a solvent. The liquid concentration may appropriately becontrolled, and the solvent may include water or an organic solvent suchas N-methylpyrrolidone, but it is not limited thereto.

In order to obtain good dispersion of the conductive material duringpreparation of the liquid of the carbon conductive material, theconductive material may be subjected to acid-treatment or a surfactantmay be further added.

The acid-treatment may be performed by dipping the conductive materialin acid such as sulfuric acid, hydrochloric acid, and so on to inducesurface oxidation, which makes good dispersion in a solvent,particularly water.

Surfactants may include non-ionic, cationic, or anionic surfactants.Examples of cationic or anionic surfactants include sulfonate (RSO₃ ⁻),sulfate (RSO₄ ⁻), carboxylate (RCOO⁻), phosphate (RPO₄ ⁻), ammonium(R_(y)H_(y)N₊: where x ranges from 1 to 3, and y ranges from 3 to 1),quaternary ammonium (R₄N⁺), betaine (RN⁺(CH₃)₂CH₂COO⁻), or sulfobetaine(RN⁺(CH₃)₂CH₂SO₃ ⁻), and examples of non-ionic surfactant includepolyethyleneoxide (R′OCH₂CH₂(OCH₂CH₂)_(n)OH), or amine compounds where Ris a saturated or unsaturated hydrocarbon group. According to oneembodiment of the present invention, the R is a C₂ to C₁₀₀₀ saturated orunsaturated hydrocarbon group and the surfactant has a molecular weightranging from 5 to 10000.

The amount of surfactant is appropriately controlled since it does notaffect the effect of the present invention.

Subsequently, the pH of the resulting mixture is controlled within arange from 3 to 4. The pH may be controlled by adding an acid or a base.Examples include acetic acid, hydrochloric acid, sulfuric acid, ammonia,and so on.

The pH-controlled second mixture is then subjected to heat-treatment.The heat-treatment is performed at from 300 to 450° C. During theheat-treatment, the water-soluble polymer disposed between the activematerial and the carbon conductive material is decomposed and moistureis also removed, and therefore, the water-soluble polymer is not presentin the resulting active material.

The entire preparation processes may be performed once, and then aftercompleting the preparation processes, the preparation processes may berepeated using another carbon-based conductive material. That is to say,after the heat-treatment, a carbon conductive material is added to theresulting product to prepare a second mixture, the pH of the secondmixture is controlled, and then heat-treated. Herein, the mixing ratio,addition amount, pH, and heat-treatment may be controlled as describedabove. The carbon-based conductive material may include carbon nanofibers, carbon nanotubes, carbon black, ketjen black, acetylene black,or activated carbon.

When the entire preparation processes are performed twice, the resultingactive material includes from 0.05 to 20 wt % and preferably from 0.3 to10 wt % of a conductive material.

An electrode is fabricated using the active material of the presentinvention as follows: an active material, a binder, and a solvent aremixed to prepare a slurry-type active material composition.

The binder attaches the active material particles to each other, andalso attaches the active materials to a current collector. The bindermay be any one or more of those generally-used materials for arechargeable lithium battery. The binder includes an organicsolvent-soluble binder or a water-soluble binder. Organicsolvent-soluble binders include polytetrafluoroethylene, polyvinylidenefluoride, polyethylene, polypropylene, polyvinylchloride,polyvinylpyrrolidone, or polyvinylalcohol.

Water-soluble binders include styrene-butadiene rubber, sodiumpolyacrylate, copolymers of propylene and C₂ to C₈ olefins, or acopolymers of (meth)acrylic acid and alkylester (meth)acrylate.

When a water-soluble binder is used, a water-soluble thickener may alsobe used to improve the binding properties of the water-soluble binder.Water-soluble thickeners include cellulose-based compounds.Cellulose-based compounds include carboxylmethyl cellulose,hydroxypropyl cellulose, hydroxypropyl methyl cellulose, hydroxyethylcellulose, hydroxypropyl ethyl cellulose, or methyl cellulose. Analkaline metal salt of the cellulose-based compound may also be used.Alkaline metals of the alkaline metal salt may include Na, K, Li, and soon. The alkaline metal salt of the cellulose-based compound may providehigh rate discharge characteristics compared to the cellulose-basedcompound.

The active material composition is applied on a current collector,dried, and then compressed to fabricate an electrode. The currentcollector generally includes an Al foil or Cu foil.

A rechargeable lithium battery including the above described positiveelectrode includes a negative electrode and an electrolyte solution.

The electrolyte solution includes a non-aqueous organic solvent and alithium salt. The non-aqueous organic solvent acts as a medium fortransmitting ions taking part in the electrochemical reaction of thebattery. The non-aqueous organic solvent may include a carbonate-based,ester-based, ether-based, or ketone-based solvent. Examples ofcarbonate-based solvents include dimethyl carbonate, diethyl carbonate,dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate,methylethyl carbonate, ethylene carbonate, propylene carbonate, butylenecarbonate or so on. Examples of ester-based solvents includeγ-butyrolactone, n-methyl acetate, n-ethyl acetate, n-propyl acetate,and so on. Ether-based solvents include dibutyl ether and ketone-basedsolvents include polymethylvinyl ketone. Carbonate-based solventsinclude mixtures of a cyclic carbonate and a linear carbonate. Thecyclic carbonate and the linear carbonate are mixed together in a volumeratio of 1:1 to 1:9, and when this mixture is used as an electrolyte,the electrolyte performance may be enhanced.

The non-aqueous organic solvent may also further include an aromatichydrocarbon-based organic solvent that is mixed with a carbonate organicsolvent. The aromatic hydrocarbon-based organic solvent may berepresented by the following Formula 17:

where R₁ to R₆ are the same or different and each is independentlyselected from the group consisting of halogens, nitro, C₁ to C₁₀ alkyls,and C₁ to C₁₀ haloalkyls, and q is an integer from 0 to 6.

Aromatic hydrocarbon-based organic solvents include benzene, fluorobenzene, chlorobenzene, nitro benzene, toluene, fluorotoluene,trifluorotoluene, xylene, and so on. When the electrolyte includes anaromatic hydrocarbon-based organic solvent, the carbonate solvent andaromatic hydrocarbon-based organic solvent are used in a volume ratiofrom 1:1 to 30:1. Within the above volume ratio, the electrolyteperformance may be enhanced.

The lithium salt supplies lithium ions in the battery for operation of arechargeable lithium battery, and facilitates transmission of lithiumions between positive and negative electrodes. Non-limiting examples oflithium salts include LiPF₆, LiBF₄, LiSbF₆, LiAsF₆, LiClO₄, CF₃SO₃Li,LiN(SO₂CF₃)₂, LiC₄F₉SO₃, LiAlO₄, LiAlOCl₄, LiN(SO₂C₂F₆)₂),LiN(C_(x)F_(2x+1)SO₂)(C_(y)F_(2y+1)SO₂) (where x and y are naturalnumbers), LiCl, Lil, or combinations thereof.

The supporting electrolytic lithium salt may be used at a 0.1 to 2.0Mconcentration. When the lithium salt concentration is less than 0.1 M,electrolyte performance may be deteriorated due to low electrolyteconductivity, whereas when it is more than 2.0M, lithium ion mobilitymay be reduced due to an increase of electrolyte viscosity.

FIG. 1 is a schematic cross-sectional view of a rechargeable lithiumbattery according to one embodiment of the present invention. Referringto FIG. 1, the cylindrical rechargeable lithium battery 1 is mainlyconstructed of a negative electrode 2, a positive electrode 4, aseparator 3 interposed between the negative electrode 2 and the positiveelectrode 4, and an electrolyte in which the separator 3 is immersed,and also includes, a cell case 5 and a sealing member 6 sealing the cellcase 5. The rechargeable lithium battery is not limited to the abovestructure, and thus the battery including a positive active material maybe fabricated in a shape of prism, pouch, or so on.

The following examples illustrate the present invention in more detail.These examples, however, should not in any sense be interpreted aslimiting the scope of the present invention.

EXAMPLE 1

A carbon nanotube conductive material was acid-treated by dipping in andout of a solution of sulfuric acid. The acid-treated carbon nanotubeconductive material was put in deionized water to prepare a liquid of anoxidized carbon nanotube conductive material.

20 g of a lithium cobalt oxide active material was added to 100 ml of agelatin aqueous solution in a concentration of 2 wt % and thereafter,filtered, preparing a lithium cobalt oxide coated with a gelatin layer.

The lithium cobalt oxide coated with a gelatin layer was added to theabove liquid of the carbon nanotube conductive material and then, thecombination was agitated. Herein, the lithium cobalt oxide was mixedwith the carbon nanotube conductive material in a weight ratio of96.5:0.5.

The acquired mixture was regulated to a pH from 3 to 4 by using aceticacid so that carbon nanotubes could be cohered together and adhered onthe surface of the lithium cobalt oxide coated with a gelatin layer.

Then, the lithium cobalt oxide having carbon nanotubes on the surfacewas oxidized at about 400° C. to remove an amount of the gelatin andabsorbed moisture, preparing an active material with a lithium cobaltoxide core and an oxidized carbon nanotube conductive shell formedaround the lithium cobalt oxide core.

The active material was mixed with an N-methylpyrrolidone organicsolvent. Then, a polyvinylidene fluoride binder was added thereto,preparing slurry. Herein, the active material was mixed with the binderin a ratio of 97:3 by wt. In the total mixture of the active materialand the binder, a conductive material was included in an amount of 0.5wt %. The prepared slurry was coated on an aluminum current collectorand compressed, preparing a positive electrode. The positive electrodewas used to fabricate a coin cell.

EXAMPLE 2

A coin cell was fabricated according to the same method as in Example 1except that lithium cobalt oxide was mixed with a carbon nanotubeconductive material in a weight ratio of 96.7:0.3.

EXAMPLE 3

A coin cell was fabricated according to the same method as in Example 1except that lithium cobalt oxide was mixed with a carbon nanotubeconductive material in a weight ratio of 96.9:0.1.

EXAMPLE 4

A coin cell was fabricated according to the same method as in Example 1except that after a lithium cobalt oxide core with a carbon nanotubeconductive shell was prepared at a weight ratio of 96.7:0.3, 0.2 wt % ofcarbon nanotube was added thereto again to prepare an active materialhaving 0.5 wt % of a carbon nanotube conductive shell in total.

EXAMPLE 5

A coin cell was fabricated according to the same method as in Example 1except that after a lithium cobalt oxide core with a carbon nanotubeconductive shell was prepared at a weight ratio of 96.7:0.3, 0.2 wt % ofcarbon black was added thereto again to prepare an active materialhaving 0.5 wt % of a carbon nanotube conductive shell in total.

EXAMPLE 6

A coin cell was fabricated according to the same method as in Example 1except that a LiFePO₄ active material was used instead of a lithiumcobalt oxide to prepare an active material, 97 parts by weight of theactive material were mixed with 0.5 parts by weight of a carbon nanotubeconductive material and 2.5 parts by weight of a polyvinylidene fluoridebinder in an N-methylpyrrolidone solvent.

EXAMPLE 7

A coin cell was fabricated according to the same method as in Exampleexcept for the following changes; an active material was prepared byusing Li_(1.2)Mo_(0.06)V_(0.85)O₂ instead of a lithium cobalt oxide.Then, 87 parts by weight of the active material was mixed with 5 partsby weight of a carbon nanotube conductive material and 8 parts by weightof a polyvinylidene fluoride binder to prepare an active materialslurry. The slurry was coated on a copper current collector to prepare anegative electrode.

EXAMPLE 8

A coin cell was fabricated according to the same method as in Example 5except for the following changes; an active material was prepared byusing a silicon active material instead of a cobalt oxide. Then, 87parts by weight of the active material was mixed with 5 parts by weightof a carbon nanotube conductive material and 8 parts by weight of apolyvinylidene fluoride binder to prepare an active material slurry. Theslurry was coated on a copper current collector to prepare a negativeelectrode.

EXAMPLE 9

A coin cell was fabricated according to the same method as in Example 8except for using an anatase-type TiO₂ active material and adding 1 partby weight of carbon nanotubes.

EXAMPLE 10

A negative active material was fabricated according to the same methodas in Example 1 except for using graphite instead of a lithium cobaltoxide as an active material and copper as a current collector.

EXAMPLE 11

A coin cell was fabricated according to the same method as in Example 1except for the following changes; an active material was prepared byusing a graphite active material instead of a lithium cobalt oxide.Then, 97 parts by weight of the active material was mixed with 0.5 partsby weight of carbon nanotubes and 2.5 parts by weight of apolyvinylidene fluoride binder to prepare active material slurry, andthe slurry was then coated on a copper current collector to prepare anegative electrode.

EXAMPLE 12

Si, Sn, Cu, and Al were treated in an arc melting method. According tothe arc melting method, the materials were mixed under an argon gasatmosphere and thereafter, fused at 1500° C. or more. The preparedSiCuAl alloy was treated using a quenching ribbon coagulation method,and an active material was prepared including Si inside a Cu—Al matrix.Herein, the quenching speed (i.e., spinning speed of a Copper roll) was3000 rpm.

The negative active material included 40 parts by weight of Si, 10 partsby weight of Sn, 44.15 parts by weight of Cu, and 5.852 parts by weightof Al.

The active material was used to prepare negative active materialaccording to the same method as in Example 1. However, the oxidationprocess at 400° C. was performed under an Ar non-active atmosphere at400° C.

EXAMPLE 13

A coin cell was fabricated according to the same method as in Example 1except for using a Li_(1.1)VO₂ lithium vanadium oxide active materialinstead of a lithium cobalt oxide active material.

EXAMPLE 14

A coin cell was fabricated according to the same method as in Example 13except for using a Li_(1.08)Mo_(0.02)VO_(0.9)O₂ lithium vanadium oxideactive material instead of a LiVO₂ lithium vanadium oxide activematerial.

EXAMPLE 15

A coin cell was fabricated according to the same method as in Example 13except for using a Li_(1.08)V_(0.9)O₂ lithium vanadium oxide activematerial instead of a LiVO₂ lithium vanadium active material.

EXAMPLE 16

A coin cell was fabricated according to the same method as in Example 1except for using a mixed active material including 28.5 parts by weightof a Li_(1.08)V_(0.9)O₂ lithium vanadium oxide and 66.5 parts by weightof artificial graphite instead of a lithium cobalt oxide activematerial.

Then, the negative active materials prepared according to Examples 1 to16 were examined and were shown to include carbon nanotubes adhered onthe surface and also, carboxyl groups, carbonyl groups, and hydroxylgroups adhered on the surface of the conductive material.

COMPARATIVE EXAMPLE 1

0.5 wt % of carbon black was used to prepare an active material coatedwith carbon black according to the same method as in Example 1.

COMPARATIVE EXAMPLE 2

A lithium cobalt oxide was mixed with a carbon black conductive materialand a polyvinylidene fluoride binder in an N-methyl pyrrolidone solvent.Herein, their mixing weight ratio was 96.5:0.5:3 by weight.

The slurry was used to prepare a positive electrode according to thesame method as in Example 1.

COMPARATIVE EXAMPLE 3

A positive electrode was prepared according to the same method as inComparative Example 2 except that carbon black was included in an amountof parts by weight.

COMPARATIVE EXAMPLE 4

0.3 wt % of carbon black was coated on the surface of a lithium cobaltoxide according to the same method as in Example 1, preparing a lithiumcobalt oxide coated with carbon black. 96.5 parts by weight of thelithium cobalt oxide coated with carbon black was mixed with 0.2 partsby weight of carbon black and 3 parts by weight of a polyvinylidenefluoride binder in an N-methyl pyrrolidone, preparing slurry. The slurrywas used to prepare a positive electrode according to the same method asin Example 1.

COMPARATIVE EXAMPLE 5

A positive electrode was prepared according to the same method as inComparative Example 1 except for preparing a LiFePO₄ active materialcoated with carbon black by using a LiFePO₄ active material instead of alithium cobalt oxide and thereafter, preparing a slurry by mixing 96.5parts by weight of the active material, 0.5 parts by weight of carbonblack, and 3 parts by weight of a polyvinylidene fluoride binder.

Comparative Example 6

A Li_(1.2)Mo_(0.06)V_(0.85)O₂ active material was coated with carbonblack, preparing an active material coated with carbon black.

A coin cell was prepared according to the same method as in Example 1except that 87 parts by weight of the active material coated with carbonblack was mixed with 5 parts by weight of a carbon black conductivematerial and 8 parts by weight of a binder in an N-methyl pyrrolidonesolvent, preparing slurry and the slurry was then coated on a coppercurrent collector.

COMPARATIVE EXAMPLE 7

A negative active material was prepared according to the same method asin Example 5 except that an active material coated with carbon black wasprepared by using a silicon active material, and then, 87 parts byweight of the active material coated with carbon black was mixed with 5parts by weight a carbon black conductive material and 8 parts by weightof a binder to prepare slurry.

COMPARATIVE EXAMPLE 8

A negative active material was prepared according to the same method asin Example 8, except that a TiO₂-anatase active material having a carbonblack conductive shell was prepared by using an anatase-type TiO₂ activematerial and 1 wt % of carbon black and then, adding 1 part by weight ofcarbon black thereto, so that conductive material might be included in atotal amount of 2 wt %.

COMPARATIVE EXAMPLE 9

A negative active material was prepared according to the same method asin comparative Example 1 except for using graphite instead of carbonblack.

COMPARATIVE EXAMPLE 10

An active material was prepared by using graphite instead of carbonblack according to the same method as in Comparative Example 1. Then, anegative active material was prepared according to the same method as inExample 1 except that 96.5 parts by weight of the active material wasmixed with 0.5 parts by weight of carbon nanotube and 3 parts by weightof a polyvinylidene fluoride binder to prepare a slurry.

COMPARATIVE EXAMPLE 11

An active material prepared according to Example 12 was used as anegative active material.

Then, the coin cells according to Examples 1 to 2 and ComparativeExample 1 were charged and discharged 40 times. Their substrates afterthe charges and discharges were photographed by a SEM. FIGS. 2 to 4respectively show the results. As shown in FIGS. 2 to 4, the coin cellsaccording to Examples 1 and 2 maintained conductive networks among thesurface of active materials. However, the one of Comparative Example 1had a crack, leading to contact defects among particles.

Next, the coin cells according to Examples 1 to 12 and ComparativeExamples 1 to 11 were examined regarding discharge capacity, 1st chargeand discharge efficiency, and 50th cycle-life efficiency characteristic.The results are shown in the following Table 1.

TABLE 1 1^(st) charge and Discharge discharge 50^(th) cycle-lifecapacity efficiency efficiency (mAh/g) (%) (%) Example 1 159 98 91Example 2 151 96 85 Example 3 138 91 72 Example 4 158 97.5 93 Example 5157 98 90 Example 6 154 97.3 89 Example 7 265 83 88 Example 8 1046 78 68Example 9 153 87 91 Example 10 355 95 91 Example 11 357 96 93 Example 12— 90% 80% Comparative Example 1 158 97.5 73 Comparative Example 2 147 9460 Comparative Example 3 114 86 35 Comparative Example 4 158 98 78Comparative Example 5 152 96.5 69 Comparative Example 6 251 78 45Comparative Example 7 1028 57 32 Comparative Example 8 151 82 78Comparative Example 9 354 94 79 Comparative Example 10 356 93 84Comparative Example 11 — 82 68

As shown in Table 1, the coin cells according to Examples 1 to 12 had apositive or negative active material electro-conductive channel with lowconductivity, which is formed by oxidized nanotube or nanofiber coatedon the surface of the positive or negative active material andaccordingly, had higher discharge capacity than the ones of ComparativeExamples 2 to 5 and 9.

In addition, in case that coin cells in general might have a crack dueto expansion of an active material, the coin cells of ComparativeExamples 1, 6 to 8, and 10 included an active material coated withcarbon black and thereby, it might be hard to maintain the shape ofelectro-conductive networks formed of static-electric coagulation ofnano beads. Even the electro-conductive networks could easily break off,failing in securing good cycle-life efficiency.

However, since the coin cells according to Examples 1 to 12 included anactive material coated with nano fiber or nanotube, they could maintaina channel among active material particles, even when fibers (or tubes)connecting active material particles were widened. Based on thesecharacteristics, they could maintain electro-conductivity inside asubstrate that may suppress resistances even if their cycle-lifeincreased and thereby, have improved cycle-life efficiency.

Furthermore, the coin cells according to Examples 14 to 16 showedsimilar results to the ones according to Examples 1 to 12.

As described above, the active material may include a conductive shellincluding a fiber-shaped or tube-shaped carbon conductive material,resulting in increased discharge capacity due to improved conductivityand/or improved cycle-life efficiency by maintaining paths betweenactive material particles during charge and discharge cycles.

While this invention has been described in connection with what arepresently considered to be practical exemplary embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments, but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

1. An improved active material for a rechargeable lithium battery,comprising: an active material selected from the group consisting oflithium alloys, lithium-containing chalcogenide compounds, andcombinations thereof; and a fiber-shaped or tube-shaped carbonconductive material directly coated on a surface of the active material,wherein the fiber-shaped or tube-shaped carbon conductive material isoxidized at its surface.